1. Field of the Invention
The present invention relates to an antenna-shared distributor and, more particularly, to an antenna-shared distributor for a dielectric line for use in a millimeter-wave radar or the like to be mounted in a vehicle and a transmission and receiving apparatus using such distributor.
2. Description of the Related Art
Millimeter-wave radars to be mounted in a vehicle are used to measure a relative velocity between a subject vehicle and another vehicle and the distance between the subject vehicle and the other vehicle. Transmission and receiving apparatus of such millimeter-wave radars are generally modules including a millimeter-wave voltage controlled oscillator, a circulator, a coupler, a mixer, an antenna and the like, and this module is mounted in the front or the back of a vehicle and used.
In FIG. 11, the relative distance and the relative velocity between a truck 500 and an automobile 600 driving in front is measured by transmitting and receiving a millimetric wave from the truck 500 by an FM-CW method. FIG. 12 is a block diagram of a millimeter-wave radar used herein. The transmission and receiving apparatus and the antenna are mounted to the front of the vehicle. A signal processing section limits a transmission and receiving apparatus in order to cause it to transmit a millimetric wave by a predetermined method, and analyzes radio waves received by the antenna in order to compute, for example, the distance between two vehicles. When the distance between the two vehicles reaches below a predetermined distance, the signal processing section controls a warning section in order to call a driver to attention.
FIG. 13 shows the construction of a transmission and receiving apparatus of the prior art, and is a schematic plan view illustrating a state in which a conductor plate which covers the top portion of a dielectric strip which will be described later is removed. In FIG. 13, reference numeral 72 denotes a circulator, with a voltage controlled oscillator 71 and a terminater 73 being disposed on two sides of the circulator 72. Reference numeral 81 denotes a dielectric resonator for transmission of a primary radiator, with a dielectric strip 74 being disposed between the circulator 72 and the dielectric resonator 81. Reference numeral 82 denotes a dielectric resonator for reception of a primary radiator, and reference numeral 85 denotes a mixer, with a dielectric strip 84 being disposed between the dielectric resonator 82 and the mixer 85. Further, a linear-shaped dielectric strip 76, dielectric strips 75 and 77 which each constitute a bent portion, and terminators 78 and 79 are disposed. A portion proximate to the dielectric strips 74 and 75 is structured as a coupler 80, and a portion proximate to the dielectric strips 84 and 77 is structured as a coupler 83. Further, dielectric lenses 86 and 87 are mounted in the upper portion of the dielectric resonators 81 and 82, respectively.
FIG. 14 is an equivalent circuit diagram of the transmission and receiving apparatus shown in FIG. 13. The voltage-controlled oscillator 71 includes a varactor diode and a Gunn diode. An oscillation signal therefrom is propagated to the dielectric resonator 81 through the circulator 72 and radiated to the outside through a window formed in the conductor plate in the upper portion of the dielectric resonator 81 and the dielectric lens 86. The circulator 72 and the terminater 73 constitute an isolator. An RF signal received via the dielectric lens 87 and the dielectric resonator 82 is propagated through the dielectric strip 84. At this time, a Lo signal (local oscillator signal) is introduced to the dielectric strip 84 by the couplers 80 and 83 and input to the mixer 85. The mixer 85 is formed of a Schottky barrier diode and generates an IF signal (intermediate frequency signal).
However, in a transmission and receiving apparatus for a millimeter-wave radar, a curved line (bent portion) must be provided in a dielectric strip, and couplers comprising two dielectric strips which are brought close to each other must be provided at at least two places within the apparatus in order to supply a part of a transmission signal as a Lo signal (local oscillator signal) to the mixer. A large area is required to construct such a coupler. In particular, when a dielectric line is formed using a nonradiative dielectric line (NRD guide), the orthogonality between the LSM01 mode and the LSE01 mode is distorted in the curved portion, and coupling occurs between the modes. Therefore, a low-loss characteristic of the line is realized only at the radius of curvature and the curved corner in a narrow range of the line. Thus, the shape of the coupler is limited, and formation of the apparatus into a small size is difficult.
Meanwhile, the aperture diameter of the antenna depends on the specifications of the transmission and receiving apparatus. When, for example, a transmission wave is within a spread of 3.5 m after 100 m in the forward direction, the beam angle is 2.degree., and the antenna aperture diameter must be approximately 170 mm in the 60 GHz band. Further, when a transmission wave is within a spread of 3.5 m after 50 m in the forward direction, the beam width angle is 4.degree.. For this reason, the antenna aperture diameter must be approximately 90 mm. In the transmission and receiving apparatus of FIG. 13, since the area where the coupler is formed together with the voltage-control oscillator and the mixer becomes larger than the antenna size, the transmission and receiving apparatus is forced to be enlarged as a whole.
Further, for the purpose of achieving a small size of the apparatus, shared use of a single antenna in transmission and reception is possible. However, in this case, a circulator for distributing a transmission signal and a receiving signal becomes necessary, and therefore a large reduction in size cannot be realized.